The invention relates to hydraulic lift systems and more particularly to controls for effecting actuation of such systems.
In hydraulic lift systems it is advantageous in some applications to use an air pressure driven motor to drive a hydraulic fluid pump which selectively supplies hydraulic fluid pressure to the hydraulic cylinder of the hydraulic lift system. An example of a prior art arrangement is illustrated in U.S. Pat. No. 4,251,055. Attention is also directed to U.S. Pat. No. 4,889,472.
In some prior art hydraulic lift systems the air motor is connected to the air supply such that it continues to run in neutral even when the hydraulic pump is not called on to supply hydraulic fluid to the lift cylinder. The air supplied to the air motor is vented to atmosphere and this results in a decrease in the air pressure in the air supply line. The hydraulic system control valve includes air pressure actuated pilot valves connected to the air supply line.
In operation of air powered hydraulic lift systems, it is common to use a hydraulic control valve which is a three position, four-way air piloted valve. When such a valve is actuated to cause operation of the lift cylinder and the lift cylinder is placed under load, the combination of the hydraulic fluid pressure in the valve and the hysteresis of the valve, may be too great to be overcome by the pilot air pressure if the air pressure in the supply line is less than a predetermined air pressure. The invention provides a hydraulic lift system including a control circuit design that uses a pilot to open an in-line air flow control valve with a dual circuit separation shuttle valve to allow sequenced operation of the control valve that changes the hydraulic fluid flow to extend or retract the cylinder component of the hydraulic lift.
The in-line valve opens when it receives a pilot signal to allow air flow to the air motor that powers the hydraulic pump. The signal comes from an air supply that is directed to the in-line valve from either side of the air control circuit used to power the four-way hydraulic valve. The air signal is controlled manually via a hand held pendant control used by the operator to send air pressure to shift the valve to advance or retract positions. The pendant is a manifold block that houses two three-way push button air valves. In operation of the hydraulic lift system, the operator opens the main air supply valve. This supplies air to a control pendant and in-line valve. The operator depresses either of the two manual air valve buttons. The air supply is delivered to the air control pilot on one side of the hydraulic valve as well as the pilot piston on the in-line valve. The hydraulic valve shifts and the in-line valve opens to cause the air supply to be allowed to reach the air motor thereby turning the hydraulic pump to supply hydraulic fluid to the hydraulic valve and the lift. The shuttle valve separates the two sides of the air pilot control circuit so that the pilot signal can come to the in-line valve from either side of the circuit without letting the signal bleed across to the pilot circuit on the opposite side of the hydraulic valve. When the operator releases the button on the pendant, the control valve returns to a neutral position and the in-line air valve closes stopping the pump. This action provides for a deadman control of the hydraulic lift.
One of the advantages derived from the control circuit arrangement is improved shifting performance of the valve. When the air motor runs, the air line supply pressure drops due to the fact that the air supply lines typically used provide a relatively small volume of air to the air motor. When the air motor is not controlled in some sequence of operation by the hydraulic valve, the air pressure that is left with the air motor running may not be enough to operate and overcome the hysteresis of the hydraulic valve. Additionally, the action of the hydraulic valve may become erratic allowing the valve to shift under lower hydraulic pressures but not at higher pressures. This may result in a variety of jack malfunctions with the most common one being a jack that will raise a load but wherein difficulties are experienced in lowering the load.
With the additional controls provided by the present invention, the air control activates from the static air line supply pressure, which is typically higher than the air line pressure when the motor is running. This insures that the control valve has enough air pressure to shift before the air motor starts. The in-line valve also functions to throttle the air to the air motor which helps maintain a predetermined air control pressure in the air control circuit. When the air pressure drops too low, the in-line valve loses the pilot signal required to maintain the valve open. As it closes, it limits the air consumed by the air motor thereby maintaining the air supply at the level required by the demand.